With recent increases in the communication traffic, demands concerning optical communication systems are also increasing. Introduction of not only an optical relay node into a basic network but also introduction of an optical communication system is actively pursued recently for local networks and, an optical communication system has also been formed for a subscriber system. Thus, optical communication systems play an important role in world-wide information networks.
To cope with the rapid increase of data communication, construction of a higher-speed and larger-capacity optical communication system that uses wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) is advancing. Introduction of an optical communication system at 40 [Gb/s] or higher has been started.
When the propagation speed reaches 40 [Gb/s] or higher, the optical pulse width of an optical signal becomes narrow, that is, several pico seconds and therefore, waveform distortion caused by slight wavelength dispersion of an optical fiber significantly degrades the propagation performance of the optical fiber. For example, when the propagation speed is increased from 10 [Gb/s] to 40 [Gb/s], dispersion tolerance becomes only 1/16. The dispersion of a propagation fiber temporally varies with temperature variation and environmental variation, and slight variations affect propagation performance.
One approach involves disposing a tunable dispersion compensator (TDC) for each channel in a receiving apparatus and controlling performance degradation caused by wavelength dispersion by adding dispersion of a sign opposite to that of dispersion accumulated in a transmission path such that the accumulated dispersion is offset.
It is considered that a more stable propagation property of a transmission path may be secured by adjusting the amount of dispersion compensation by a TDC corresponding to temporal variation of wavelength dispersion of the transmission path. For example, TDCs such as those of an etalon type, a virtually imaged phased array (VIPA) type, and a fiber Bragg grating (FBG) type have been developed as TDCs that can vary the amount of dispersion compensation (see, e.g., Optoelectronic Industry and Technology Development Association, “Group Delay Ripple Measurement Method for Tunable Dispersion Compensators—Technical Paper”, Oct. 9, 2008).
However, the above conventional approach has a problem in that an optical signal is degraded due to a group delay ripple in a group delay property of a tunable dispersion compensator. More specifically, in a tunable dispersion compensator whose transmission band is sufficiently wider than an effective band of the group delay property, an optical signal input into the tunable dispersion compensator is degraded due to a group delay ripple outside the effective band when a band occupied by the optical signal overlaps a band outside the effective band of the group delay property of the tunable dispersion compensator.
In a tunable dispersion compensator that is conventionally used such as that of the VIPA type, the transmittance in bands outside an effective band is low and therefore, a component of a band affected by a group delay ripple in an optical signal is attenuated. Therefore, the influence of the group delay ripple on the optical signal is slight. Hence, no attention has been paid to the degradation of an optical signal due to a group delay ripple of a tunable dispersion compensator.
In contrast, in a tunable dispersion compensator whose transmission band is sufficiently wide relative to a dispersion band such as an etalon TDC, a component in each band outside an effective band is not substantially attenuated. Therefore, it is known that an optical signal is degraded due to an influence of a group delay ripple in a tunable dispersion compensator such as that of the etalon type whose transmission band is sufficiently wide relative to a dispersion band.
A band occupied by an optical signal is widened when the bit rate of the optical signal is high and therefore, it is difficult to establish an effective band of the group delay property in a tunable dispersion compensator to the extent that the effective band may cover the band occupied by the optical signal. Hence, degradation of an optical signal due to a group delay ripple in the group delay property of a tunable dispersion compensator becomes significant as the bit rate of the optical signal becomes high.
It is generally known that a trade-off relation exists between the amount of wavelength dispersion compensation by a tunable dispersion compensator and an effective band of a group delay property, and it may be considered that the effective band of the group delay property of the tunable dispersion compensator is widened by providing multiple tunable dispersion compensators each having a small wavelength dispersion compensation amount and a wide effective band of the group delay property. In this case, the band occupied by the optical signal input into the tunable dispersion compensator may also be adapted to not overlap any band outside the effective band in the group delay property of the tunable dispersion compensator. However, in this case, problems arise such as increases in insertion loss, the size of the apparatus, and the cost therefor caused by providing the tunable dispersion compensator in plural.